1. Field of the Invention
The present invention relates to a pigment dispersion composition, an inkjet recording method, and a method for producing a compound.
2. Description of the Related Art
In recent years, as an image recording method for forming an image on a recording medium based on image data signals, an inkjet recording method has drawn attention. The inkjet recording method has advantages of making small noise and being able to record a high-definition image at low running costs by ejecting extremely small liquid droplets.
According to the inkjet recording method, printing can be performed not only on plain paper but also on a recording medium not absorbing liquid such as a plastic sheet or a metal plate. However, in order to increase the printing speed and improve the image quality, the time taken for drying or curing is required to be shortened. As one of the inkjet methods that meet such requirements, there is a recording method in which an ink that can be cured in a short time by being irradiated with actinic energy rays is used as an inkjet recording ink. According to this method, by curing ink droplets by the irradiation of actinic energy rays immediately after printing, a sharp image can be formed. The curable inkjet recording ink described above is required to have high pigment dispersibility and temporal stability of the dispersed pigment, such that a high-definition image having excellent color developability is formed, and the ink is stably ejected.
In order to impart a vivid color tone and high coloring power to the ink, it is essential to finely pulverize the pigment. Particularly, in a case of the inkjet recording ink, the ejected ink droplets exert a great influence on the sharpness of an image. Therefore, the ink droplets are ejected in a small amount, and it is essential to use fine particles having a size smaller than a film thickness of the cured ink film formed of the ink. However, if the pigment particles are more finely pulverized to obtain high coloring power as described above, it is difficult to disperse the fine particles, and aggregates are easily generated. Furthermore, there is also a problem in that the viscosity of the ink increases due to the addition of an excess of dispersing agent. Both the generation of the pigment aggregates and the increase in viscosity of the ink exert a negative influence on the ink ejectability. Accordingly, it is not preferable to use an ink which causes pigment aggregation, viscosity increase, and the like as an inkjet recording ink.
In a case of an actinic energy ray-curable type ink jet recording ink (hereinafter, simply referred to as a “curable type ink”), curability for forming a sharp image or film properties after curing are also important factors. In a case of the curable type ink, the ink is rapidly cured by being irradiated with actinic energy rays so as to secure sharpness of an image. Therefore, it is not preferable to use a pigment dispersing agent which hinders curing or a pigment dispersing agent which contains a solvent as a non-curable component, because such an agent reduces curability and causes bleeding or reduction in productivity.
Particularly, in a case of the curable type ink containing a solvent as a non-curable component, the curing sensitivity is reduced due to a plasticizing effect, and the concentration of the ink changes during storage due to the volatilization of the solvent component. Furthermore, the curable type ink causes ejection failure and a so-called blocking phenomenon in which in a case where printed matters (those obtained by performing printing on recording media) are stored by being stacked on each other, a printing surface of a printed matter adheres to a non-printing surface of another printed matter and thus the printing surface and the non-printing surface in the adherend are contaminated. Therefore, it is preferable to use a pigment dispersing agent not containing an organic solvent as far as possible.
In this respect, there is a demand for a curable type ink which can excellently disperse a finely pulverized pigment, has excellent dispersion stability, and uses a pigment dispersing agent not containing an organic solvent as far as possible. As the pigment dispersing agent, various compounds have been suggested.
For example, a pigment dispersing agent is known which is obtained by reacting a compound, which has a hydroxyl group and a pigment-adsorbing partial structure, with a polymer, which is selected from aliphatic polyester, poly(meth)acrylate, and polyether and has a hydroxyl group or a primary amino group, and an isocyanate compound which has two or more functional groups in a polymerizable compound (for example, see JP2009-233486A). By synthesizing polyurethane in a polymerizable compound such as an acryl monomer, the content of the organic solvent in the pigment dispersing agent is reduced.
Furthermore, a pigment dispersing agent for a printing ink is known which includes a main chain, which is composed of a polyurethane resin and/or a polyurea resin having one or more kinds of groups selected from the group consisting of an amino group, an imino group, and a monovalent or divalent aromatic hydrocarbon group, and a side chain which is composed of a graft polymer including a polyether skeleton and/or a polyester skeleton (for example, see JP2008-13725A). By performing a step of distilling away the organic solvent, the content of the organic solvent in the pigment dispersing agent is reduced.